Things that I know I need to purchase is a standon or walk behind mower between 36" - 52" deck widths. Possibly a 60" zero turn mower, atleast a 16" dual axel trailer.

Questions
Mower, should I buy a a ZTR and a trailer and maybe a used walk behind? Or should I buy a nice stander and a walk behind? Or just a nice walk behind? I am going to be mostly just doing residential to start so what should I really go for?

Trailer, I do not have a space to house my trailer so I am thinking I should pick up a used covered trailer 16' + around $3k?

why not start with a used walkbehind mower like a bobcat or scag roughly 32 to 36" and add a sulky to it that way you can ride around the yards. You can find good condition belt driven bobcat 36" wb's for good deals. I have seen them on craigslist selling for 500 dollars and just needed 1 or 2 replacement belts. With proper engine care they can last a forever with just replacing belts. Sounds like you have solid trimmers and a blower but I would add a push blower to your arsenal for leaves and debris.
I have a 4x8 and 5x8 trailer both open trailers. I am going to convert the 4x8 into a leaf trailer for the fall and also use that for mulch jobs during summer and spring. I would go with an open trailer at first until you know as a fact that your going to get enough buisness.
Also, starting with the walk behind you will be able to guage if your going to have large enough lawns where you would need a riding ztr. These are just my opinions.

whatever mower you do end up purchasing, if it is belt driven, make sure you keep at least one replacement belt of each one onboard with you in the truck or on the trailer. If one breaks in the field and you have to either go home or to a repair shop, its buying time you could be working. Also, keep an extra spool of trimmer line with you. Goes quick when you slam concrete retaining walls by accident.

Congrats on the accounts, good work!!
My opinion,
If you are confident that the Deere can last a season or 2, then you have a solid 38" mower already -MAYBE I would pick up a used 48 or 52" WB, depends on how many more you pick up-- At your rate you may be one of those 40-50 account newbies by Spring! But if your solo and the Deere runs well, and you have some money to invest, I might start looking into a good deal on a used 350-550 range dump truck. You have some time so you could catch a good deal if research. An enclosed trailer is the way to go, and 16' is nice. I have a similar setup to yours and I only have a 12' enclosed. If you organize things (racks etc.) the stuff you have now will easily fit. 14' is good too.
I guess my point is that if you have 10K to spend, a dump truck ($6-8K) and an enclosed trailer (14' = $2K) will carry you and all your current equipment plus some material to the job with no added overhead, and at the rate which you are marketing, you can pick up another mower during or after the season. Plus you'll better know what size you should get.
Again, this is just how my mind works so do what you feel is right, but I think for you, a dump is a money maker!!!
Damn I wish I had $10K!!!!!

I appreciate the advise, the father in law would have no stake in the company what so ever he would just be a helping hand if I got behind or needed some extra help here and there. I am planning on staying completely solo if I can. Thanks